1. Field of the Invention
This invention relates to a process for refining the aqueous acid mixture obtained from the mixed acid nitration of aromatic compounds. More particularly, it is directed to a process for refining the sulfuric acid utilized in the nitration reaction so that the end product will be substantially free of contaminant material and color.
2. Description of the Prior Art
U.S. Pat. No. 3,856,673 relates to a process for refining sulfuric acid utilized in the mixed acid nitration of aromatic compounds, e.g. benzene and toluene. As disclosed in the prior art section, it was common practice to steam strip the spent acid mixture obtained from the nitrator to remove volatile organic and inorganic impurities e.g. nitrous acid, sulfur compounds, and nitrous oxides. It was pointed out that the nonvolatile organic compounds remained in the sulfuric acid and had to be bled from the system or as disclosed in the patent, oxidized with an oxidizing agent to volatile products.
U.S. Pat. No. 3,204,000 discloses a process for producing nitrotoluene in a continuous nitration process. The pertinent portion of this patent lies in its disclosure of the purification technique for the aqueous acid mixture. The patentee points out in column 1, lines 50-60 that substantial quantities of deleterious nitrous acid are produced during nitration particularly when the dinitrotoluene concentration is high. At column 2, lines 5-20, it is pointed out that nitrous acid can be removed by increasing the nitric acid concentration but that undesired side reactions resulting in a discolored nitrotoluene product and acid attack of the stainless steel reactor results. Another technique utilized for removing nitrous acid involved passing air through a separator and stripping with dilute nitric acid.
U.S. Pat. No. 2,849,497 discloses a process for producing nitrobenzene with particular emphasis on refining the crude nitrobenzene produced. In that patent, the aqueous acid mixture obtained from the nitrator is separated from crude nitrobenzene in a settling tank. The spent acid is passed to a denitrator where it is contacted with feed benzene to form a crude benzene-nitrobenzene product which is then passed to the nitrator. A portion of the sulfuric acid product from the denitrator then is contacted with the crude nitrobenzene for the purpose of scrubbing residual nitric acid dissolved therein and then it is returned to the denitrator. Another portion of the sulfuric acid from the denitrator is sent to a sulfuric acid concentrator where it is reconstituted for reuse.